Defense weapon systems typically include provision for target identification before active engagement of the weapon. An IFF system may, for example, be used to distinguish between friendly and unfriendly aircraft in order to permit action to be taken against an unfriendly aircraft. In operation the IFF system must be able to provide accurate target discrimination in the presence of a plurality of aircraft. Typical existing IFF antenna systems use antennas providing a fan beam of fixed vertical orientation which can be mechanically rotated in azimuth. Such antennas provide only azimuth discrimination between potential targets, so that all aircraft within an azimuth beamwidth are commonly interrogated, regardless of differing aircraft altitudes. In certain defense weapons system applications using this type of azimuth discriminating antenna, the weapon system cannot be activated against a low altitude unfriendly aircraft so long as a friendly aircraft is within the same azimuth beamwidth. This result obtains even if the friendly aircraft is at a much higher elevation. As a result, weapons system effectiveness may be limited under high traffic conditions involving a number of aircraft.
It is therefore an object of this invention to provide pencil beam antenna systems having antenna patterns capable of distinguishing between multiple target positions in both elevation and azimuth.
An additional object is to provide antenna systems operating with both a sum mode antenna pattern and a new type of ring mode antenna pattern extending in the direction of the sum mode beam centerline and having a zero to 360 degree progressive phase characteristic around such centerline.
Another object is to provide antenna systems wherein a sum mode signal and a ring mode signal are processed to provide an antenna pattern providing new and improved operating characteristics. Further objects are to provide new and improved antenna systems which avoid one or more shortcomings of prior antenna systems.